In the past decade, digital imaging capabilities have been integrated into a wide range of devices, including digital cameras and mobile phones. Recently, device manufacturers have introduced devices integrating multiple digital imaging sensors in one device. A wide range of electronic devices, including mobile wireless communication devices, personal digital assistants (PDAs), personal music systems, digital cameras, digital recording devices, video conferencing systems, and the like, can make use of multiple imaging sensors to provide capabilities and features to their users. These capabilities include higher dynamic range imaging, panoramic imaging, and stereoscopic (3D) imaging applications such as 3D movies.
To provide these new advanced applications, the multiple cameras of a device are calibrated to work together. Applications such as stereoscopic imaging require at least two cameras to focus on the same subject. Furthermore, during the recording of a 3D movie, the focal distance of the camera sensors will frequently change, requiring each camera to react simultaneously and in unison with the other camera to maintain a clear image.
To achieve a synchronized focus, devices with a plurality of imaging sensors are usually calibrated during the manufacturing process. The device is placed into a special “calibration mode” on the manufacturing line, with the imaging sensors pointed at a target image designed to ensure consistent focus. Each camera of the device is then focused on the target image and its focus position recorded by a calibration station. The two recorded focus positions are then stored in a non-volatile memory of the device. Alternatively, an offset may be generated between the two focus positions instead and stored in the non-volatile memory. When the product is later purchased and used, the mobile device consults the prerecorded calibration data to set lens positions during use.
This calibration process has several disadvantages. First, it takes up time during the manufacturing process, increasing the cost of the device. Second, any calibration data produced during manufacturing is static in nature. As such, it cannot account for changes in lens calibration as the device ages. For example, the piezo electric motors controlling the focus position in many of the small imaging sensors embedded in today's mobile devices may lose efficiency over time. In some imaging sensor embodiments, for example those using a voice coil type actuator, this results in less camera lens adjustment for a given applied voltage as the camera sensor ages. Other embodiments may require more electronic pulses or commands to achieve a particular lens position as the sensor ages. To the extent an imaging device calibration relies on a fixed differential voltage between two imaging sensors, misalignment of imaging sensor focal distances are likely after significant device usage. This divergence in focal distances causes final images to be less clear, ultimately effecting customer satisfaction.
Ensuring a precise focus is further complicated by variations in lens focal distance with temperature and orientation. Calibrations taken at the factory are generally performed at either a horizontal or vertical orientation. Multiple orientations are typically not calibrated. Nor is device temperature typically a consideration in factory calibrations. Performing these multiple calibrations at different temperatures or orientations would add to manufacturing time, and the cost of equipment needed to control the temperature of the device during these calibration steps may also add significant cost.